This invention relates to television apparatus, and more particularly, to a system for providing a television display which portrays the path of motion of a moving object in an otherwise static scene wherein a limited number of decaying images appear behind the latest image of the moving object.
Commonly assigned patent applications Ser. Nos. 864,405 filed Dec. 27, 1977 now U.S. Pat. No. 4,179,704, and 944,236 filed Sept. 20, 1978, both entitled "Television System for Displaying and Recording Paths of Motion", describe systems for providing a television display which portrays the motion of an object in a scene during a selectable time interval such that, in addition to its present position being shown, a plurality of earlier positions commencing with the start of the time interval, are also depicted. This effect is achieved by storing a reference field of video representing the scene at the start of the desired action, comparing picture elements of subsequently arriving video with those of the reference field, tagging and storing those picture elements measurably different from corresponding picture elements in the reference field, and displaying the real time video with the substitution of those tagged and stored picture elements which comprise the successive images of the moving object. While similar results are theoretically possible if subsequently arriving television frames are compared with a stored reference frame, because one cannot effectively display a frozen frame of video, where image motion is involved, without causing inter-field flutter, the data preferably is stored in a one field format, and as this data is recirculated, this single field is interlaced with itself. Thus, the term "field" uniformly used in the present description and claims is intended to also encompass "frame", unless obviously inapplicable or unless specific exception is made. The system provides television special effects with most of the benefits of stroboscopic photography, and additionally provides functions previously unavailable in film or television. These results are obtained without special lighting setups or black backgrounds, as are required for stroboscopic photography, and are available both live and in replay during the normal course of coverage of sporting or other events.
Although the systems described in the copending applications provide display of successive positions of a moving object moving over a selectable time period in an otherwise static scene, in situations where it is desired to display a multiplicity of sometimes overlapping paths of motion of an object, for example, repeated flights of a tennis ball back and forth across the net, the separated images of the ball in the earlier occurring trajectories would tend to obscure the images recorded in later flights, resulting in a confusing overlap of image positions which would give little useful or aesthetic information on any given trajectory, including the latest. When watching a televised tennis match, particularly in situations where there is a sustained volley, the viewer will have little interest in already completed volleys and is really interested only in the trajectory of the ball back and forth across the net in the latest one or two volleys, with major interest in the most recent trajectory. Thus, a system for displaying multiple, possibly crisscrossing, paths of motion of a moving object desirably should delete or "erase" from the display those images of the moving object representative of an eariler trajectory of the moving object so as to display only spaced apart images of more recent successive positions of the moving object. A particularly desirable display is one in which the image representing the current or present position of the object has a predetermined brightness behind which appear a limited number of spaced apart images of continuously and progressively decaying intensity.